The present invention relates to prevention of erroneous operations of a dynamic brake circuit of a motor to be driven by an inverter comprised of semiconductor switching devices.
To stop inverter drive motors, which are represented by permanent magnet synchronous motors, in a case of emergency, a method called xe2x80x9cdynamic brakexe2x80x9d has been employed in which the operation of the inverter is stopped, power generated from the power supply lines of the motor that rotates under inertia is short-circuited with a brake resistor and consumed as heat energy, and the energy is absorbed for braking.
FIG. 5 is a circuit diagram of a conventional brake for an inverter drive motor.
In this figure, to anode 26 at the output of the bridge circuit having six diodes 25 connected to a three-phase AC power supply, one end of smoothing resistor 27 is connected, and smoothing capacitor 29 is connected between the other end of this smoothing resistor 27 and cathode 28 at the output of the bridge circuit. In parallel with this smoothing capacitor 29, three pairs of semiconductor switching devices 30 such as transistors and thyristors, each two of which are connected together in series, are connected. The intermediate connecting points of these semiconductor switching devices 30 are connected to the power supply lines 32 of the motor 31, and the motor is driven at a phase corresponding to the switching timing of the semiconductor switching devices 30.
On the other hand, to brake this motor 31, a series circuit is provided, which consists of a three-phase bridge rectifier circuit including diodes 33 connected to the power supply lines 32, thyristor 34 as a semiconductor braking switch, and brake resistor 35. A snubber circuit in which snubber resistor 36 and snubber capacitor 37 are connected in series is connected to this thyristor 34 in parallel. In the figure, ignition current limiting resistor 38, photocoupler 39 consisting of a photodiode and a photothyristor, bias capacitor 40, and bias resistor 41 comprise an ignition control circuit of the thyristor 34.
In such a conventional inverter, in order to cause the motor 31 to make an emergency stop, for example, when continuity of the semiconductor switching devices 30 is interrupted to stop power supply to the motor 31, and the photodiode of the photocoupler 39 is caused to emit light and the photothyristor is caused to electric-insulatingly ignite, a signal is applied to the gate of the thyristor 34, so that the thyristor 34 ignites and makes continuity. Thereby, electricity that has been generated from the motor 31 and sent to the power supply lines 32 flows to the diodes 33, thyristor 34, brake resistor 35, diodes 33, and power supply lines 32, and then heat is generated and absorbed by the brake resistor 35. Thereby, the motor 31 is rapidly braked.
However, in a conventional inverter, in the switching operations of the semiconductor switching devices 30 when operating the inverter, if the rate of voltage change dv/dt is excessively great, the critical OFF voltage rise rate of the thyristor is exceeded, the photothyristor and thyristor 34 of the photocoupler 39 are caused to erroneously ignite, and the inverter outputs, that is, the power supply lines 32 are short-circuited although there is no request for braking.
In order to prevent this problem, the capacitance of the snubber circuit must be made sufficient to suppress the rate of voltage change dv/dt, or a semiconductor braking switch with a sufficiently great critical OFF voltage rise rate must be selected. Therefore, the circuit becomes large and complicated, and cost for the parts increases.
Therefore, the object of the invention is to provide a safe brake for an inverter drive motor in which the rate of voltage change dv/dt generated by the operation of the semiconductor switching devices of the inverter is not directly applied to the semiconductor braking switch.
In order to achieve the abovementioned object, according to the first aspect of the invention, a dynamic brake circuit, which comprises a series circuit including resistors for converting loaded electric energy into heat and semiconductor switching devices, which are connected in series, and a snubber circuit including a capacitor connected in parallel to the semiconductor switching devices, further comprises a charging circuit for charging electricity in the capacitor prior to driving to load.
Furthermore, according to the second aspect of the invention, in a semi conductor inverter, which comprises a first rectifier for rectifying alternating currents, a smoother for smoothing the output of the first rectifier, and an inverter part for switching the output from the smoother at a desired timing by the first semiconductor switching device, and further comprises a dynamic brake circuit consisting of a second rectifier for rectifying the output from the inverter part and a series circuit including the first resistance and second semiconductor switching device that are connected between the output terminals of the second rectifier, and a snubber circuit including a capacitor connected in parallel to the second semiconductor switching device, the inverter is provided with a charging circuit for charging electricity in the capacitor before the inverter starts an inverter operation.
Furthermore, according to the third aspect of the invention, the charging circuit is comprised of a second resistance connected between the anode side of the smoother and anode side of the dynamic brake circuit, and a third switching device.
Moreover, according to the fourth aspect of the invention, in place of the third switching device, a part of the first semiconductor switching devices is commonly used.
According to the fifth aspect of the invention, in the inverter according to the second aspect of the invention, which has flywheel diodes connected in antiparallel to the first semiconductor switching device, the flywheel diodes are commonly used in place of the diodes comprising one arm of the second rectifier.